Electrolytic cell for purifying aqueous solutions of alkali metal hydroxides

ABSTRACT

Electrolytic apparatus is described for purifying an aqueous solution of an alkali metal hydroxide containing an impurity of a soluble heavy metal complex comprised of a heavy metal cation and a plurality of anions. 
     For example, an aqueous solution of sodium hydroxide containing a soluble heavy metal complex, such as mercuric polysulfide, [HgS] +  S - , is charged to the electrolytic chamber of an electrolytic cell. An electric current is employed to reduce heavy metal cations to a separate phase in elemental form, and simultaneously to oxidize anions to a separate phase in elemental form, and to form a slurry of the separate phases in elemental forms in the aqueous solution. 
     The slurry is removed from the electrolytic chamber and the phases of elemental forms are separated from the purified aqueous solution. The purified alkali metal hydroxide solution is sold commercially or otherwise utilized.

This is a division, of application Ser. No. 3,151, filed Jan. 15, 1979,now U.S. Pat. No. 4,189,362.

The present invention relates to apparatus utilized with a particularmethod for purifying aqueous solutions of alkali metal hydroxides byremoving heavy metal contaminants and chemical groups which formcomplexes with heavy metal contaminants. The present invention may beutilized in apparatus employed for recovery of mercury compounds fromchemical plant effluents and natural waters. More specifically, thepresent invention relates to the specific apparatus utilized with aparticular electrolytic method for purifying an aqueous solution ofsodium hydroxide by removing mercury and sulfide contaminants containedtherein.

Heavy metal contaminants, such as mercury, are objectionable in mostchemical processes and preparations involving sodium hydroxide. Variouschemical and electrolytic methods have been employed to remove heavymetal contaminants, such as mercury, from aqueous solutions of sodiumhydroxide.

U.S. Pat. No. 3,502,434, issued to John Buchanan MacMillan on Mar. 24,1970, discloses a process for removing mercury from mercury cell liquorwherein the mercury is absorbed and then recovered from a composite bedof particulate material such as polyethylene shreds and a metal such asnickel or stainless steel.

U.S. Pat. No. 3,764,495, issued to Joel P. Guptill and Gary W. Foley onOct. 9, 1973, discloses a method for removing mercury from mercury cellliquor wherein a mercury containing vapor is passed through a causticpotash solution in the presence of a small amount of a reducing agent.

U.S. Pat. No. 3,853,722, issued to Donald T. Rigler and Knut J. Johnsenon Dec. 10, 1974, discloses that sodium hydroxide from the denuder of amercury cell may be electrolyzed in an electrolytic cell employing aporous cathode to reduce the concentration of mercury.

In spite of these methods, a need has remained in the industry for amethod of sufficiently purifying aqueous solutions of sodium hydroxide,such as mercury cell caustic without employing additional and oftencostly process equipment.

OBJECTS

It is a primary object of the present invention to provide inexpensive,simple, and efficient apparatus for use in a particular method forpurifying aqueous solutions of alkali metal hydroxides.

It is another object of the present invention to provide inexpensive,simple, and efficient apparatus for use in a particular method forremoving trace levels of mercury from aqueous solutions of sodiumhydroxide produced as mercury cell caustic in mercury cells.

A further object of this invention is to provide apparatus useable withan aqueous solution of sodium hydroxide which contains less than about0.3 part per million of mercury by weight.

BRIEF DESCRIPTION OF THE INVENTION

The foregoing objects of the invention are accomplished in the processof this invention by electrolyzing an aqueous solution of an alkalimetal hydroxide containing an impurity of a soluble complex comprised ofa heavy metal cation and a plurality of anions, in an electrolytic cellhaving an electrolytic chamber containing therein an anode and a cathodeseparated from each other by a gap distance.

The aqueous solution of alkali metal hydroxide is charged to theelectrolytic chamber and an electric current is passed between the anodeand cathode to reduce heavy metal cations and transfer them to aseparate phase in elemental form, to oxidize the complex anions to aseparate insoluble phase in elemental form, and to form a slurry ofthese phases of elemental forms in the aqueous solution.

The slurry is removed from the electrolytic chamber and the phases ofelemental forms are separated from the aqueous solution of alkali metalhydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a typical electrolytic cell useful incarrying out the process of the present invention.

FIG. 2 shows an oblique view of a typical cathode assembly useful incarrying out the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

More in detail, as shown in the typical cell of FIG. 1, circularelectrolytic cell 8 is comprised of cell bottom 9, cylindrical side 10secured thereto, a cell cover 11 secured to cylindrical side 10, andcathode assembly 12 contained within electrolytic chamber 13.

Cathode assembly 12 is comprised of rotatable shaft 14 and rotatableelement 15 affixed to shaft 14.

Cell bottom 9 and cylindrical side 10 are of continuous fabrication.Cell cover 11 is attached to cylindrical side 10 by placing gasketmaterial 16 atop cylindrical side flange 17.

Flange 18 of cell cover 11 is positioned atop gasket material 16. Bolts19 and 20 are inserted through flange 18 of cell cover 11, gasketmaterial 16 and cylindrical side flange 17, and are tightened.

Shaft 14 extends downward through cell bottom 9 and is insulatedtherefrom by bottom thrust bushing 22 suitably supported in cell bottom9 which also prevents outflow of any aqueous solution contained inelectrolytic chamber 13.

Shaft 14 extends upward through cell cover 11 and is insulated therefromby top thrust bushing 23 which also prevents outflow of aqueous solutioncontained in electrolytic chamber 13.

A cathode assembly vertical support member 24 is affixed to shaft 14 andrides on top of thrust bushing 23.

The cathode assembly 12 is connected externally to a rotational drivingmeans 25, such as an air motor of sufficient size and capacity to rotatethe cathode assembly 12 at desired speed. An insulator coupling 26 isemployed to connect shaft 14 with the rotational driving means 25. Thespeed of rotation of the cathode assembly 12 is in the range from about50 to about 1000 and preferably in the range from about 100 to about 750revolutions per minute.

Aqueous solution enters electrolytic chamber 13 through inlet 30, coverscell bottom 9, rotatable element 15, and substantially fillselectrolytic chamber 13.

If desired, any aqueous solution contained within electrolytic chamber13 may be removed through outlet 31.

Electric current is supplied to electrolytic cell 8 by attachingelectrical leads (not shown) to conductive connections on theelectrolytic cell 8.

For example, a positive lead (not shown) is attached to conductive anodeconnection 32 affixed to cylindrical side 10.

For the cathode, a flexible contact brush 33 is employed to contactshaft 14. Contact brush 33 is attached to top thrust bushing 23 bycathode pin 34. Current to the cathode assembly is achieved by attachinga negative lead (not shown) to cathode pin 34. Current flows throughcathode pin 34 to contact brush 33 to shaft 14 of cathode assembly 12 torotatable element 15.

In a preferred embodiment, the gap distance between cell bottom 9 andthe rotatable element 15 is maintained uniform at all points facing oneanother. For maximum exposure of the electrolytic surface, the face ofrotatable element 15 should be parallel to cell bottom 9. If desired,the rotatable element 15 to cell bottom 9 distance may be varied tomaintain optimum cell current efficiency.

The gap distance employed is in the range from about 0.1 to about 2.0and preferably in the range from about 0.2 to about 1.0 centimeters.

Although the preferably shape of the rotatable element 15 is a circulardisc, it is understood that the construction of the rotatable element 15may be either solid, felt, mesh, foraminous, expanded metal, or anyother design.

The diameter of the disc employed as the rotatable element 15 willnecessarily be less than the internal diameter of electrolytic cell 8.In general, the diameter of the disc employed will be less than theinternal diameter of the particular electrolytic cell employed in therange from about 0.1 to about 0.5, and preferably in the range fromabout 0.2 to about 0.4 centimeter.

If desired, the thickness of the disc may be varied. Generally, thethickness of the disc is in the range from about 0.1 to about 0.5 andpreferably in the range from about 0.2 to about 0.4 centimeter.

It is preferred that the contents of electrolytic chamber 13 be highlyagitated during electrolysis. Fluted vanes 35 and 36 are located onrotatable element 15 as shown in FIG. 1. Fluted vanes 35 and 36 aresmall sections or lips of rotatable element 15 which are partially cutfrom the rotatable element 15 and then bent to protrude from therotatable element 15.

The speed of rotation of the cathode assembly 12 is in the range fromabout 50 to about 1000 and preferably in the range from about 100 toabout 750 revolutions per minute.

As the rotatable element 15 is rotated, for example, counterclockwise asshown by the arrow in FIG. 1, the fluted vanes 35 and 36 providedagitation of the aqueous solution, such as sodium hydroxide, through therotatable element 15, so that a high degree of agitation is maintainedwithin electrolytic chamber 13.

FIG. 2 shows an oblique view of a typical cathode assembly 12 comprisedof shaft 14 and circular rotatable element 15. Fluted vanes 35, 36, 37and 38 are located on rotatable element 15 as shown. As cathode assembly12 is rotated, for example, in the direction indicated by the arrow, forexample, counterclockwise, any solution within the electrolytic chamberis deflected by fluted vanes 35, 36, 37 and 38 and pumped throughapertures 39, 40, 41 and 42 in rotatable element 15.

Any convenient means for maintaining a high degree of agitation withinelectrolytic chamber 13, FIG. 1, may be employed, for example, apropeller or series of propellers, suitably positioned withinelectrolytic chamber 13 or, for example, attached to cathode assembly12.

In the same preferred embodiment, the cell bottom 9 and cylindrical side10 are of continuous fabrication and are employed as an anode. Ifdesired, anodes may be employed within electrolytic chamber 13, and thecell bottom 9 and cylindrical side 10 insulated therefrom.

Examples of material which may be employed as an anode include thoseselected from a group consisting of nickel, platinized titanium,platinized tantalum, platinized platinum, and nickel deposited on anysubstrate capable of bonding with nickel such as nickel on type 316stainless steel, nickel on type 317 stainless steel, and mixturesthereof.

In general, any material that is capable of evolving oxygen withoutcorrosion in aqueous sodium hydroxide solution, and capable of effectingthe electrolytic oxidation of an anion hereafter described to theelemental form of that anion, may be employed as anode material ofconstruction.

Examples of material which may be employed as a cathode assembly 12include those metals which readily amalgamate with mercury. Cathodematerials of construction are chosen from a group consisting of copper,gold, silver, and deposits of copper, gold and silver on any substratecapable of bonding with copper, gold and silver, such as deposits ofcopper, gold, and silver on type 316 stainless steel and deposits ofcopper, gold and silver on 317 stainless steel and mixtures thereof.

In general, any material that is capable of effecting the electrolyticreduction of the cation of the heavy metal hereafter described to theelemental form may be employed as a cathode material of construction inthe process of this invention.

A plurality of anodes and a plurality of cathode assemblies 12 may beemployed in the process of this invention, for example, a series ofrotatable elements 15 may be affixed to shaft 14 within the electrolyticchamber 13.

In addition, any geometric pairing of anodes and cathode assemblies 12may be employed.

The anode and the cathode assembly 12 may be stationary in the processof this invention, however, it is preferred that the electrolytic cellemploy at least one rotatable electrode, preferably a rotatable cathodeassembly.

One skilled in the art will recognize that the electrolytic cellemployed in carrying out the process of this invention may be acommercially available or a custom-built electrolytic cell of a size andelectrical capacity capable of economically producing the desiredpurified product.

Aqueous solutions of alkali metal hydroxides which may be purified bythe process of this invention include aqueous solutions of sodiumhydroxide, potassium hydroxide, lithium hydroxide and mixtures thereofhaving a concentration in the range from about 5 to about 95% metalhydroxide by weight.

As used throughout the description and claims, the term "heavy metal" isdefined as any metal which is capable of combining with a complexinggroup to form a heavy metal complex of the form [MC]⁺ X⁻, where M is anyheavy metal selected from a group consisting of mercury, arsenic,antimony, bismuth, tin and mixtures thereof, where C and X are a"complexing group" or any electron donor either ionic or nonionic andcapable of combining with M, the heavy metal previously described, toform a complex of the type [MC]⁺ X⁻, C and X may be the same ordifferent complexing group.

A typical aqueous solution of alkali metal hydroxide containing heavymetals and complexing agents as contaminants which may be purifiedaccording to the process of this invention is an aqueous solution ofsodium hydroxide produced as an effluent from the mercury cell known asmercury cell caustic. The same purification may be applied to otheraqueous solutions containing contaminant heavy metals, but forsimplicity, the process of this invention is hereafter described moreparticularly with respect to the purification of mercury cell caustic.Such description is not to be construed as limiting the usefulness ofthe invention or the scope of the appended claims.

The aqueous sodium hydroxide purified by the apparatus and process ofthis invention is produced in a mercury cell.

Mercury cells are comprised of an enclosed, elongated trough whichslopes slightly toward one end and have a cathode and anodes. Thecathode is a flowing layer of mercury which is introduced at the higherend of the cell and flows along the bottom of the cell toward the lowerend. The anodes are generally comprised of foraminous metal orrectangular blocks of graphite suspended from conductive lead-ins sothat the bottom of the anodes is spaced a short distance above theflowing mercury cathode. An aqueous electrolytic solution, for example,a brine of sodium chloride is fed to the upper end of the cell, coveringthe anodes and flowing concurrently with the mercury. The impressedelectric current passing through the electrolytic solution between theanodes and the mercury cathode liberates chlorine at the anodes andsodium is dissolved in the mercury as an amalgam. The sodium mercuryamalgam flows from the lower end of the cell to a decomposer where it iscontacted with water to form hydrogen, elemental mercury and an aqueoussolution of sodium hydroxide, commonly known an mercury cell caustic.

The mercury cell caustic is thereafter separated from the hydrogen andmercury.

The pH of the aqueous solution of mercury cell caustic is in the rangefrom about 7 to about 14 and preferably in the range from about 8 toabout 13.

The concentration of sodium hydroxide in the mercury cell caustic is inthe range from about 25 to about 75 and preferably in the range fromabout 40 to about 60% by weight sodium hydroxide.

In addition to containing sodium hydroxide, the mercury cell caustic maycontain small concentrations of heavy metals, such as mercury.

In abnormal mercury cell plant operation, a small percentage of heavymetal, such as mercury, may be reacted to the ionic state and combinedwith other elements, for example, sulfur, in the form of sulfide, S⁼, toform compounds, such as insoluble mercuric sulfide, HgS. Heavy metalsulfides, such as mercuric sulfide, are relatively insoluble in themercury cell caustic and may be recovered therefrom by conventionalmethods, for example, centrifugation, filtration, coalescence,absorption and the like.

Without being bound by theory, it is believed that the relativelyinsoluble heavy metal sulfur compounds, such as mercuric sulfide,previously described may further combine with additional sulfur to forma soluble heavy metal polysulfide complex of the form [HgS]⁺ S⁻. Forexample, insoluble mercuric sulfide, HgS, may further combine withadditional sulfur in the form of sulfides to form a soluble mercurypolysulfide complex of the form [HgS]⁺ S⁻. The mercuric polysulfidecomplex is highly soluble in the mercury cell caustic in sharp contrastto the highly insoluble mercuric sulfide, HgS.

With the apparatus employed in the process of this invention, mercurycell caustic of the type described above contaminated with a complex ofa heavy metal polysulfide such as mercuric polysulfide, [HgS]⁺ S⁻, andhaving an equivalent concentration of mercury in the range from about0.5 to about 1000 parts per million of mercury by weight, is charged toelectrolytic chamber 13 of an electrolytic cell of the type previouslydescribed, or other convenient design.

However, the apparatus of this invention may be employed with theprocess disclosed herein for mercury cell caustic having equivalentconcentrations of mercury above about 1000 parts per million mercury byweight as well.

In operation of the process of this invention, direct current issupplied to the cell and a voltage is impressed across the cellterminals. The cell voltage employed is in the range from about 1.0 toabout 4.0 and preferably in the range from about 2.0 to about 3.0 volts.

Without being bound by theory, it is believed that as an electriccurrent is passed between the anode and the cathode, that on thecathode, heavy metal cations such as Hg⁺⁺ are reduced to a separatephase, Hg°, in elemental form, and anions such as S⁼, are liberated fromthe mercuric polysulfide complex. It is believed that the cathodereaction is represented by Equation (1):

    2e.sup.- +[HgS].sup.+ S.sup.- →Hg°+S.sup.=   (1)

where Hg° represents elemental mercury.

Without being bound by theory, it is believed on the anode, that anions,such as sulfide, S⁼, are oxidized to a separate phase, S°, in elementalform. It is believed that the anode reaction is represented by Equation(2):

    S.sup.= →S°+2e.sup.-                         (2)

where S° represents elemental sulfur; however, further oxidation ispossible with increased cell potential at the anode to give:

    S°+O.sub.2 ⃡SO.sub.2

    SO.sub.2 +xH.sub.2 O⃡SO.sub.4.sup.-2 +4H.sup.+ +(x-2)H.sub.2 O+2e.sup.-

    SO.sub.3.sup.-2 +2OH.sup.- ⃡SO.sub.4.sup.-2 +H.sub.2 O+2e.sup.-

Completion of the reactions represented in Equation (1) and Equation (2)results in the formation of a slurry of the phases of elemental forms,such as Hg°, and S°, in the aqueous solution.

The reaction period required for the completion of the reactionrepresented in Equation (1) is dependent on the electrolytic cellgeometry but is generally in the range from about 2 to about 40 andpreferably in the range from about 3 to about 30 minutes.

The operating temperatures of the electrolytic cell is in the range fromabout -10° to about 80° and preferably in the range from about -5° toabout 40° C.

The pressure of the electrolytic cell is essentially atmospheric,although subatmospheric or superatmospheric pressure may be employed.

The resulting slurry of phases in aqueous sodium hydroxide is withdrawnfrom the electrolytic cell 8 through, for example, outlet 31, FIG. 1.

The phases of elemental forms are separated from the aqueous solution byany suitable solid-liquid separation technique, such as by filtration,centrifuging, settling, and the like. Filtration is the preferred formof solid-liquid separation although any other suitable solid-liquidseparation technique may be employed. When filtration is employed, it ispreferable to employ an inorganic or organic filter precoat such as adiatomaceous earth, a cellulosic-based material, or a silica-basedmicroporous material.

The phases of elemental forms separated from the aqueous solutioncontain a minimal amount of residual aqueous solution and may bedisposed of as waste, or otherwise utilized.

Further separation of the phases of elemental forms may be employed, forexample, by retorting to separate elemental mercury from elementalsulfur.

The purified aqueous solution of alkali metal hydroxide produced by theseparation contains concentrations of heavy metals, such as mercury, ofless than about 0.3 part per million by weight mercury, and is soldcommercially or otherwise utilized.

The electrolytic cell of the process of this invention can be operatedon a batch or flow-through system. In the latter system, the aqueoussolution is continuously circulated to and from an external storagevessel. In addition, at least a portion of the aqueous solution isseparated from the electrolytic chamber and external storage vessel.

Typically, the heavy metal complex is formed in situ in the aqueoussolution of alkali metal hydroxide purified in the process of thisinvention.

However, if desired, additional complexing anions may be added to theaqueous solution of alkali metal hydroxide to permit removal of theheavy metal cation.

Either inorganic or organic compounds complexing agents which providethe additional complexing ions may be employed.

Although sulfur in the form of sulfide ions is typically present inaqueous solution of alkali metal hydroxides, such as mercury cellcaustic, as described above, other inorganic complexing agents which maybe employed in the process of this invention include (i) alkali metalthiosulfates such as sodium thiosulfate, potassium thiosulfate andmixtures thereof, (ii) alkaline earth metal thiosulfates such as calciumthiosulfate, magnesium thiosulfate and mixtures thereof, (iii) alkalimetal thiocyanates such as sodium thiocyanate, potassium thiocyanate andmixtures thereof, (iv) alkaline earth metal thiocyanates such as calciumthiocyanate, magnesium thiocyanate and mixtures thereof, (v) alkalimetal oxylates such as sodium oxylate, potassium oxylate and mixturesthereof, (vi) alkaline earth metal oxylates such as calcium oxylate,magnesium oxylate and mixtures thereof, (vii) alkali metal cyanides suchas sodium cyanide, potassium cyanide and mixtures thereof, (viii)alkaline earth metal cyanides such as calcium cyanide, magnesium cyanideand mixtures thereof. (ix) alkali metal sulfides such as sodium sulfide,potassium sulfide and mixtures thereof, (x) alkaline earth metalsulfides such as calcium sulfide, magnesium sulfide and mixturesthereof, (xi) alkali metal orthophosphates such as sodiumorthophosphate, potassium orthophosphate and mixtures thereof, (xii)alkaline earth metal orthophosphates such as calcium orthophosphate,magnesium orthophosphate and mixtures thereof, (xiii) alkali metalpyrophosphates such as sodium pyrophosphate, potassium pyrophosphate andmixtures thereof, (xiv) alkaline earth metal pyrophosphate such ascalcium pyrophosphate, magnesium pyrophosphate and mixtures thereof,(xv) alkali metal triphosphates such as sodium triphosphate, potassiumtriphosphate and mixtures thereof, (xvi) alkaline earth metaltriphosphates such as calcium triphosphate, magnesium triphosphate andmixtures thereof, (xvii) mixtures of alkali metal thiosulfates, alkalineearth metal thiosulfates, alkali metal thiocyanates, alkaline earthmetal thiocyanates, alkali metal oxylates, alkaline earth metaloxylates, alkali metal cyanides, alkaline earth metal cyanides, alkalimetal sulfides, alkaline earth metal sulfides, alkali metalorthophosphates, alkaline earth metal orthophosphates, alkali metalpyrophosphates, alkaline earth metal pyrophosphates, alkali metaltriphosphates and alkaline earth metal triphosphates.

Other complexing agents include ammonia, propylene diamine,triethanolamine, citric acid, glyconic acid, oxalic acid, glycine,α-α'-dipyridyl and mixtures thereof.

Preferred inorganic complexing agents include those selected from agroup consisting of sulfur, sodium sulfide, potassium sulfide, sodiumpyrophosphate, sodium triphosphate, and mixtures thereof.

Organic complexing agents include compounds selected from the groupconsisting of trimethylamine, tributylamine, triethylamine,tripropylamine and mixtures thereof.

Other organic complexing agents include compounds selected from thegroup consisting of trimethylphosphine, triethylphosphine,tripropylphosphine, tributylphosphine and mixtures thereof.

Another example of a suitable family of organic complexing agentsinclude compounds selected from a group consisting of trimethylarsine,triethylarsine, tripropylarsine, and mixtures thereof.

Preferred organic complexing agents include complexing agents selectedfrom a group consisting of ethylene diamine, ethylene diaminetetraacetic acid, ethylene diamine diacetic acid and mixtures thereof.

It is recognized that inorganic and organic complexing agents may beemployed simultaneously, for example, these complexing agents selectedfrom a group consisting of sulfur, sodium sulfide, potassium sulfide,sodium pyrophosphate, sodium triphosphate, ethylene diamine, ethylenediamine tetraacetic acid, ethylene diamine diacetic acid, and mixturesthereof.

Although reference has been made primarily to the treatment ofrelatively high concentrations of caustic solutions such as thoseobtained directly from the operation of electrolytic mercury cells, themethod is also applicable to the removal of mercury contained in weakcaustic solutions and effluents.

This method is also applicable to the removal of organic and inorganicmercury from chemical plant effluents and other aqueous solutions suchas water.

The present invention may be employed to purify brine containingcontaminants of the nature which are picked up from a mercury cell, andto the purification of brine regardless of contaminant source. The brinewill generally be an aqueous solution of sodium chloride. However, thepresent invention may be employed to purify brines or aqueous solutionsof other alkali metal hydrides such as potassium chloride, lithiumchloride, rubidium chloride, and cesium chloride. The process of thepresent invention would include the removal of mercury, which is presentas an ion, from the aqueous brine solution of any metal chloride whichis sufficiently electropositive such that hydrogen gas, rather than themetal, is liberated at the cathode.

Brines containing in the range from about 110 to about 350 grams ofsodium chloride per liter of solution may be treated effectively.However, brine containing greater or lesser amounts of sodium chloridemay be purified using the present invention. The brine will, inactuality, rarely contain less than about 50 grams of NaCl per liter.The brine, for practical reasons, preferably should be acidic, having apH in the range from about 1 to about 5, although brine that is alkalinemay also be purified by the present invention.

The following example is presented to define the invention more fullywithout any intention of being limited thereby. All parts andpercentages are by weight unless indicated otherwise.

EXAMPLE 1

About 300 milliliters of an aqueous solution containing about 44.8 partsper hundred sodium hydroxide, about 2.1 parts per million dissolvedmercury, and about 1.8 parts per million dissolved sulfur, was purifiedby electrolysis in an electrolytic cell.

Without being bound by theory, it is believed that the dissolved mercuryand dissolved sulfur were present in the aqueous solution of sodiumhydroxide as a highly soluble mercuric polysulfide complex.

The electrolytic cell had a design similar to that of FIG. 1. The anodewas a circular nickel plate about 0.25 centimeter thick, about 2.5centimeters wide, and was bent in the shape of a rectangle about 6.25centimeters in width and about 10 centimeters in height.

The cathode was a circular copper disc of design similar to that of FIG.2. The cathode was comprised of a 0.16 centimeter thick copper sheethaving four fluted vanes and was about 5.75 centimeters in diameter andabout 0.16 centimeter in thickness.

The cathode was attached to about 0.65 centimeter circular rotatablecopper shaft.

The cell cathode had an exposed area of about 19 square centimeters. Thecathode shaft protruded through and was insulated from the top of thenickel anode.

The anode and cathode were contained in 500 milliliter standardlaboratory glass flask.

The cathode disc was positioned parallel to the bottom of the nickelanode at a distance of about 0.32 centimeter from the nickel anode.

At the top of the container, the rotatable cathode was secured in aliquidtight fashion and insulated from the nickel anode by employing apolytetrafluoroethylene bushing placed between the cathode shaft and thenickel anode.

Electrical connections were made to the cell container body and to therotatable cathode. The positive terminal of a direct current electricsupply (about 2 volts) was directly connected to the container body andbrush.

A silver impregnated graphite brush means was employed to connect anegative current lead to the rotatable cathode shaft insulated from andexternal to the cell body.

About 300 milliliters of a solution previously described was added tothe 500 milliliter standard laboratory glass flask containing the anodeand cathode.

The electric current was turned on and the solution was electrolyzed atabout 25° C., and at a constant voltage of about 2.0 volts. The cellcurrent averaged about 0.32 amperes.

The cathode disc was rotated at about 600 revolutions per minute. Thegap between the cathode disc and the cell bottom was about 0.25centimeter.

After about 3 minutes, the electrolysis was stopped. The entire solutionwas removed from the cell and filtered through a laboratory filterprecoated with a diatomaceous earth to separate the precipitated solidsfrom the purified solution.

The purified solution was found by analysis to contain about 45.0 partsper hundred sodium hydroxide, about 0.12 part per million mercury, andabout 1.2 parts per million sulfur with the remainder essentially water.

What is claimed is:
 1. An electrolytic cell for electrolyzing an aqueoussolution of an alkali metal hydroxide containing an impurity of asoluble heavy metal complex, comprised of(a) a conductive bottom, (b)conductive sides secured to said conductive bottom, said sides having aninsulated flange at the top thereof, and forming in combination withsaid conductive bottom a first electrode, (c) a cell cover positionedatop said insulated flange to form an electrolytic chamber, (d) an inletfor feeding said solution into said cell and an outlet for removingpurified solution from said cell, (e) a rotatable assembly comprisedof(1) at least one rotatable shaft, (2) at least one second conductivemetal electrode secured to said shaft and positioned adjacent to butspaced apart from said conductive bottom, (3) said second conductivemetal electrode having at least one fluted vane with at least oneadjacent aperture therein so that when rotated said fluted vane agitatessaid aqueous solution while the aqueous solution is deflected throughsaid aperture towards the conductive bottom, and (f) means formaintaining a potential difference between said first conductive metalelectrode and said second electrode.
 2. The electrolytic cell of claim1, wherein said shaft extends through and is insulated from said cover.3. The electrolytic cell of claim 2, wherein said shaft extends throughand is insulated from said bottom.
 4. The electrolytic cell of claim 3,wherein said first metal conductive electrode is employed as a cathode.5. The electrolytic cell of claim 3, wherein said second metalconductive electrode is employed as a cathode and said bottom and saidsides are employed as an anode.
 6. The electrolytic cell of claim 5,wherein said rotational driving means is an air motor.
 7. Theelectrolytic cell of claim 6, wherein the distance between said firstconductive metal electrode and said conductive bottom is maintained by asupport member attached to said shaft above said cell cover.
 8. Theelectrolytic cell of claim 7, wherein said side has a cylindrical shape.9. The electrolytic cell of claim 8, wherein said bottom and side are ofcontinuous fabrication of conductive metal.
 10. The electrolytic cell ofclaim 9, wherein said anode is comprised of a material selected from agroup consisting of nickel, platinized titanium, platinized tantalum,platinized platinum, nickel on type 316 stainless steel, nickel on type317 stainless steel, and mixtures thereof.
 11. The electrolytic cell ofclaim 10, wherein said assembly is comprised of a material selected froma group consisting of copper, gold, silver, deposites of copper, gold,and silver on type 316 stainless steel, deposits of copper, gold, andsilver on 317 stainless steel and mixtures thereof.